Hydraulic systems of work machines may include one or more hydraulic circuits to move one or more work implements and/or to provide propulsion for the machine itself. Each actuator (e.g., a hydraulic cylinder) and/or each drive component (e.g., wheels, tracks, etc.) may be operated with its own dedicated hydraulic circuit. Pressure may build within a hydraulic circuit due to various influences, such as equipment failure or physical resistance to actuation. The pressure within the hydraulic circuit should be regulated to avoid over-pressurizing the hydraulic system.
In a work machine drive train, a hydrostatic transmission may be used in lieu of a mechanical transmission. A hydrostatic transmission may include a variable displacement hydraulic pump and a hydraulic motor, which may also have variable displacement. By varying the displacement of the pump, the amount of fluid pumped to the hydraulic motor may be controlled. Fluid pumping through the hydraulic motor may cause it to spin an output shaft to thereby move a drive mechanism, such as a wheel or track. By varying the ratio between the displacement of the pump and the displacement of the motor, the speed at which the motor is spun may be controlled.
When the movement of a drive mechanism is inhibited by some sort of external resistance, pressure may build within the hydraulic circuit that drives the drive mechanism. For example, a hydraulic circuit that functions to drive the wheels on a wheel loader may experience a rise in pressure when the loader pushes on something that is heavy or substantially immovable, like a large pile of earth. When the loader meets the resistance of the large pile of earth, the forward travel of the loader may be slowed or stopped. The slowing or stopping of the loader may cause the drive wheels to also be slowed or stopped, which, in turn, slows or stops the hydraulic motor that turns the wheels. As the hydraulic motor is slowed or stopped, the flow of fluid through the motor may be substantially inhibited. However, the variable displacement pump may continue to pump fluid to the hydraulic motor. Thus, the continued flow of fluid from the pump can elevate the pressure in the circuit by pumping additional fluid to the motor when the motor does not permit the fluid to flow through. This additional pressure should be relieved and/or prevented to avoid damage to the hydraulic system and possibly other components of the work machine.
A work implement hydraulic system may include an actuator, such as a hydraulic cylinder, to create motion instead of a hydraulic motor. When a work implement meets resistance (e.g., an excavator bucket hitting a rock), the cylinder extension may be stopped. However, the hydraulic pump may continue to pump fluid to the cylinder in an attempt to extend it. The continued flow of fluid may elevate the fluid pressure in the circuit. As discussed above, this pressure build up should be relieved and/or prevented to avoid damage to various systems and components of the work machine.
One way to relieve this kind of pressure build up is with a cross-over relief (COR) valve, which may permit hydraulic fluid to flow (i.e., cross over) from the high pressure side of the circuit over to the low pressure side. This type of pressure relief valve may effectively prevent spikes in pressure. However, continued flow across a COR valve can cause significant heating of the hydraulic fluid due to the pressure drop of the fluid as it passes from the high pressure side to the low pressure side of the system. If fluid is allowed to flow across a COR valve for an extended period of time, the fluid may be heated to levels that may cause damage to system components and/or the fluid itself. Thus, the COR valves may only be suitable for use over short periods of time, (e.g., to prevent sudden spikes in pressure).
Another common way to provide pressure relief is with a pressure override (POR) system. A POR system acts to reduce the displacement of the variable displacement pump, and thus reduce the amount of fluid being pumped to the hydraulic motor (or implement actuator). Hydraulic systems may include POR systems in addition to COR valves. Historically, work machines have been provided with mechanical POR systems, such as spool-type valves that are actuated by the pressure of the fluid itself.
Systems have been developed that utilize an electronic POR. For example, U.S. Pat. No. 5,525,043, issued on Jun. 11, 1996 to Lukich (the '043 patent), discloses a POR system for work implement hydraulics. The system of the '043 patent controls pump displacement based on measurements of pump discharge pressure. The '043 patent also controls pump displacement based on an underspeed control, which is operable to restrict pump output when the load on the engine is greater than the engine is capable of meeting.
The present disclosure is directed toward one or more improvements to existing electronic POR systems.